Such an access protocol is already known in the art, e.g. from the article `Local distribution in computer communications`, by J. F. Hayes, published in IEEE Communications Magazine, March 1981, pp. 6-14, more specifically p. 11 thereof where it is called probing. The idea of probing is to poll for user stations having to transmit messages, i.e. wanting access to the main station, and to poll these user stations in a group rather than one at a time. In successive steps the number of user stations in this group is reduced until a single access wanting user station is possibly found,.this station being then allowed to transmit its message. Thereafter a new probing operation is started. The access request signal transmitted by each of the user stations of the access group is a noise signal indicating that it wants access to the main station. Upon receipt of this signal the main station reduces the number of user stations in the access group by means of the control signal. If no noise signal is received a new access group is probed. Thus, it is clear that after each access group reducing step it may happen that no access wanting user station remains in the new, reduced, access group, in which case the former, original, access group is to be reduced to another access group.